The short answer
Apart from simple wear, the biggest outside causes of roof damage are wind, which lifts and slips coverings, frost and trapped water, which split tiles and slates, overhanging trees and moss, foot traffic on fragile coverings, and poor earlier repairs. Most do their harm slowly and out of sight, so a quick look after bad weather is worth the effort.
Wind and storms
Wind is the most obvious attacker. Strong gusts get under the edges of a roof, at the verge and eaves, and lift tiles and slates that have lost their fixings or sit on tired nails. Ridge and hip tiles take the brunt, because they stand proud of the slope. Modern practice under BS 5534:2014 adds mechanical fixings to the bedding, but plenty of older ridges still depend on mortar that has crumbled with time.
Wind does not need to blow anything off to do harm. Repeated flexing loosens fixings, and a covering that rattles in a gale has often already started to slip. Flying debris is the other risk: branches, fence panels and loose items from neighbouring gardens can crack a slate or dent flashing in seconds.
Once a storm has passed, stand back in the garden or street, binoculars if you have them, and scan for breaks in the tile pattern, fragments lying on the path or in the downpipe shoe, and ridge tiles that no longer sit in a straight line. If you find any, avoid climbing up to check and ask for a survey. Our page on a roof that is still leaking after a repair may also help if a patch has failed.
Frost, water and the slow damage of the seasons
Water does damage by getting into small places and then expanding. A damp tile or slate that freezes overnight swells at its edges, and repeated freezing and thawing flakes the surface and can split the piece. Older clay and concrete tiles that have absorbed water over the years are the most open to this, as are soft stone slates on traditional roofs.
Standing water makes it worse. Blocked gutters and clogged valleys hold water against the edge of the covering, and fascia boards and rafter ends that stay wet start to rot from the outside in. A rotten fascia can then drop the gutter, which sends rain running down the wall instead of into the drain.
Heat plays a part as well, particularly on flat roofs, where sun can harden and crack felt and sealants, and expansion and contraction opens up laps and seams. None of this is fast, which is why seasonal checks matter more than a single inspection.
Trees, moss and what grows on the roof
Trees damage roofs in several ways. Branches that rub on the surface wear away the face of tiles and slates in windy weather, a heavy limb can fall in a storm, and leaves drop into gutters and valleys, where they rot into a damp layer that holds water and feeds moss. Overhanging growth also keeps part of the roof shaded and wet for much of the year.
Moss grows in that damp and its roots grip the surface. A thick mat of moss seldom makes a hole by itself, yet it holds water against tile and slate, blocks the gaps that let rain run off, and can lift the edges of pieces as it spreads. Take care with how it is removed. Scraping by hand is gentle; a jet wash can strip the face off tiles, and MyBuilder's roof cleaning guide warns that this method may damage delicate tiles. Taking moss off also leaves the roof exposed again if the shade or damp that caused it remains.
Trimming back branches, clearing the gutters and cutting away ivy near the eaves removes most of the cause.
People, aerials and poor previous repairs
A roof is built to shed water, not to be walked on. Natural slate cracks under a boot, clay tiles snap, and fibre-cement or older cement sheets can give way entirely. The damage often shows up weeks later as a slow leak, long after the person who caused it has left. Satellite dish fitters, gutter cleaners, chimney sweeps and anyone retrieving a ball are the usual visitors.
Earlier repairs are the other big source. A blob of mastic over a crack, a bitumen patch on a failing flashing or a few tiles replaced with a different type can seem to work for a season. Then water finds its way beneath the patch, and the leak reappears somewhere new. Quick fixes can also hide a problem so well that it cannot be diagnosed.
Birds, squirrels and other animals add to it by pulling at ridge ends, nesting in eaves and loosening tiles. Both nesting birds and bats have legal protection, so if you find either in the eaves, leave them undisturbed and mention it before any work is planned.
Our guide to DIY roof repair explains where home fixes help and where they cause the next fault.
Spotting the damage and what to do next
A walk around the house tells you plenty. Look for missing, cracked or slipped tiles, sagging lines in the ridge, loose or lifted flashing, debris in the gutters and moss that has built up on one slope. Inside, watch for stains on the ceiling, damp patches in the loft and daylight at the edges of the covering.
Where damage is small, the repair is usually small too. A few replacement tiles, a section of re-bedded ridge or fresh flashing can put it right, and that is the kind of job we recommend first. A larger fault, such as widespread slipping or decayed timber, calls for more, and after the survey we explain which of the two you are facing, showing the cracked slate or rotted batten in our pictures and setting the price out on paper. Work comes with a ten-year workmanship guarantee in writing.
How long a repair lasts depends on the cause, which is why we look at what attacked the roof as well as what failed. Our page on how long a roof repair lasts goes into that, and common roof repairs covers the usual fixes. For help, see our roof repairs service.